This invention relates to the formation of polycarbonate films, including the formation of apertures through polycarbonate films.
Polycarbonate is a colorless thermoplastic polymer, i.e., polycarbonate softens when heated and hardens when cooled. Polycarbonate is commonly used in applications which take advantage of its outstanding impact resistance and toughness, such as molded helmets, battery cases, bottles and packaging, and in applications which also demand optical transparency, such as bullet-proof and safety glass, eyewear, compact discs and automobile lenses. In thin-film form, polycarbonate is used for a variety of applications ranging from precision filters to electron-emitting devices.
Polycarbonate membranes used as commercial filters are described in the 1990 Nucleopore(copyright) Laboratory Products Catalog, Costar Corp., 1990, pp. 3, 8 and 9. The membranes are created by subjecting stretched, crystalline polycarbonate film to irradiation, followed by etching to form pores. The Costar process is similar to that disclosed in Price et al., U.S. Pat. No. 3,303,085. The thickness of commercial membrane filters is typically 6 to 11 xcexcm.
Bassiere et al., PCT Patent Publication WO 94/28569, disclose how thin polycarbonate layers are used in manufacturing electron-emitting devices. In one embodiment, Bassiere et al. provide a polycarbonate layer over a sandwich consisting of an upper conductor, an insulator and a patterned lower conductor. The multi-layer structure is irradiated with heavy ions to create radiation tracks through the polycarbonate layer. The tracks are etched to form pores through the polycarbonate layer down to the upper conductor. Using suitable etchants, the pore pattern in the polycarbonate layer is transferred to the upper conductor and then to the insulator, after which conical electron-emissive elements are formed in the resulting openings in the insulator.
Bassiere et al. indicate that the thickness of their polycarbonate layer is approximately 2 xcexcm. This is significantly less than the thickness of the commercial polycarbonate membrane filters in the Costar product catalog. While Bassiere et al. specify that the polycarbonate layer in their structure can be created by spin coating, Bassiere et al. do not provide any further information on how to make the polycarbonate layer.
Macaulay et al., PCT Patent Publication WO 95/07543, disclose a similar fabrication technique in which electron-emissive features in an electron-emitting device are defined by way of charged-particle tracks formed in a track layer. Polycarbonate is one of the materials that Macaulay et al. consider for the track layer. The thickness of the track layer in Macaulay et al. is 0.1 to 2 xcexcm, typically 1 xcexcm. Consequently, the thickness of the track layer in Macaulay et al. is typically less than that of the polycarbonate layer in Bassiere et al. by a factor of up to twenty.
Kanayama et al, European Patent Specification 500,128 B1, application published Aug. 26, 1992, describes a polycarbonate resin utilized in forming a solid polycarbonate film. The polycarbonate resin consists of copolycarbonate formed with repetitions of two different carbonate repeat units. The polycarbonate film is created by dissolving the copolycarbonate in a non-halogenated solvent such as toluene, xylene, or ethylbenzene, forming a liquid film of the resulting solution over a substrate, and drying the liquid film.
The solid polycarbonate film of Kanayama et al may have enhanced mechanical strength. However, the film does not appear particularly suitable for receiving a fine pattern of small generally parallel apertures created by etching along the tracks of energetic charged particles that pass through the film. For example, the carbonate (CO3) groups in the repeat units do not appear to have significant free radical stabilization which would facilitate etching along the charged-particle tracks.
As film thickness is reduced, it becomes progressively more difficult to make high-quality polycarbonate films. Controlling and maintaining the uniformity of film thickness and other properties, such as density, becomes harder. Structural and compositional defects also become more problematic in very thin polycarbonate films. It would be desirable to have a method for making a thin polycarbonate film whose thickness and other physical properties are highly uniform, especially a thin polycarbonate film in which a fine pattern, such as a group of small generally parallel apertures, is to be formed. It would also be desirable to have a method for providing small parallel apertures through the film, particularly for use in defining openings in the gate layer of a gated electron emitter.
The present invention involves the preparation and usage of polycarbonate films. More particularly, the invention furnishes properties and compositions for a polycarbonate-containing liquid chemical formulation from which a thin polycarbonate film of highly uniform thickness can be made. The invention also furnishes processing techniques for making the polycarbonate film. Apertures are created through a so-prepared polycarbonate film by etching along substantially parallel charged-particle tracks. The aperture-containing polycarbonate film is typically employed in fabricating a gated electron-emitting device.
The liquid chemical formulation of the invention is formed from polycarbonate material dissolved in a suitable liquid, preferably one capable of dissolving the polycarbonate material to a concentration of at least 1% by mass of the liquid formulation at 20xc2x0 C. and 1 atmosphere. The liquid preferably contains a principal solvent consisting of at least one of pyridine, a ring-substituted pyridine derivative, pyrrole, a ring-substituted pyrrole derivative, pyrrolidine, a pyrrolidine derivative, chlorobenzene, and cyclohexanone. The liquid may include a cosolvent, different from the principal solvent, for modifying one or more properties of the liquid formulation.
Aside from the liquid and the polycarbonate material, the present liquid chemical formulation may be provided with one or more other constituents such as a water scavenger. To the extent that any other such constituent is present in the liquid formulation, each other such constituent is normally a minor component compared to the polycarbonate material. That is, the polycarbonate material is normally present in the liquid at a higher mass fraction than any other constituent present in the liquid.
The polycarbonate material typically includes copolycarbonate whose molecules each contain two or more different monomeric carbonate repeat units. Each carbonate repeat unit is formed with a carbonate (CO3) group and another group, normally a hydrocarbon group. The copolycarbonate normally constitutes at least 5%, typically more than 50%, by mass of the polycarbonate material.
Use of copolycarbonate leads to a polycarbonate film having properties that are highly advantageous when apertures are created in the polycarbonate film by etching along tracks formed by energetic charged particles. Each charged-particle track consists of a zone of damaged polycarbonate material in which the energy of one of the particles causes the polycarbonate molecules along the particle""s path to cleave (undergo scission). A polycarbonate molecule typically cleaves along certain of its carbonate groups as decarboxylation occurs. Carbon dioxide is released from the molecule during decarboxylation. Apertures are created along the charged-particle tracks by removing the damaged polycarbonate material with etchant that attacks the remnants of the cleaved polycarbonate molecules much more strongly than the uncleaved polycarbonate molecules.
Each polycarbonate molecule in the damaged polycarbonate material need not be cleaved into a large number of small parts for apertures to be created in the polycarbonate film by etching along the charged-particle tracks. Etchants are available which can selectively remove remnants of polycarbonate molecules cleaved at a relatively small number of locations, e.g., less than 10, typically 2-5, without significantly attacking uncleaved polycarbonate molecules. When apertures are to be created through a polycarbonate film by etching along charged-particle tracks, it is thus adequate for the polycarbonate molecules to have the property that each molecule cleaves most readily at only a relatively small number of locations when struck by energetic charged particles.
The homolytic bond cleavage energy in a carbonate repeat unit of a polycarbonate molecule normally reaches a minimum at a location along the repeat unit""s carbonate group. There is invariably a difference in minimum homolytic bond cleavage energy among the different carbonate repeat units in a molecule of copolycarbonate. Consequently, copolycarbonate molecules can be configured to have the foregoing advantageous molecular cleavage property.
More particularly, each copolycarbonate molecule contains a primary carbonate component and a further carbonate component. The primary carbonate component is formed with repetitions of a primary carbonate repeat unit. The further carbonate component is formed with repetitions of one or more further carbonate repeat units different from the primary carbonate repeat unit.
Each further carbonate repeat unit has a lower minimum homolytic bond cleavage energy than the primary carbonate repeat unit. Accordingly, each further repeat unit undergoes decarboxylation, and accompanying molecular scission, more readily than the primary repeat unit. The number of carbonate groups along which a copolycarbonate molecule cleaves most readily when struck by an energetic charged particle is thus less than the total number of carbonate groups in the molecule.
The primary carbonate components of the molecules of copolycarbonate in the polycarbonate material of the present liquid chemical formulation normally constitute more than 50%, preferably more than 80%, by mass of the copolycarbonate. Taking note of the fact that bisphenol is a readily available and relatively inexpensive hydrocarbon, the primary repeat unit of each copolycarbonate molecule preferably consists of bisphenol A carbonate. Because each further repeat unit in such an implementation of copolycarbonate cleaves more readily than the bisphenol A carbonate repeat unit, the copolycarbonate cleaves more readily at acceptable locations than polycarbonate material formed solely with bisphenol A carbonate repeat unit. By implementing the copolycarbonate in this way, the polycarbonate material in the present liquid chemical formulation yields a relatively inexpensive polycarbonate film having a fully adequate molecular cleavage property when apertures are to be created through the film by etching along charged-particle tracks.
At least one carbonate repeat unit in the polycarbonate material, especially the copolycarbonate, preferably has free radical stabilization. When molecules of the polycarbonate material undergo scission due, for example, to being struck by energetic charged particles, the free radical stabilization inhibits the remnants of the cleaved polycarbonate molecules from combining with one another or with other material. The ability of the polycarbonate material to maintain the pattern generated by the charged particles or other cleavage-causing phenomenon is thereby enhanced.
Manufacture of a polycarbonate film in accordance with the invention is accomplished by first providing a liquid chemical formulation variously having the properties described above. Water in the liquid formulation can cause undesired scission of the polycarbonate molecules. As a result, the liquid formulation is normally prepared in such a way as to strongly avoid the presence of water. For this purpose, a water scavenger is typically employed. The water scavenger is typically introduced into the liquid prior to dissolving the polycarbonate material in the liquid.
A liquid film of the present liquid chemical formulation is formed over a substructure. Various techniques, such as extrusion coating, can be utilized to create the liquid film. The liquid film is further processed to remove volatile components. The material remaining after such processing is a solid, largely polycarbonate film. Depending on the constituency of the liquid chemical formulation, the polycarbonate film may include, as minor components, one or more other non-volatile constituents of the liquid formulation and/or their reaction products. Importantly, the polycarbonate film is of highly uniform thickness, especially when the average film thickness is in the range of 0.1 xcexcm to 2 xcexcm.
As indicated above, apertures are created in the polycarbonate film by subjecting the film to charged particles and then etching along the charged-particle tracks. In a typical application, an electrically non-insulating layer of the substructure is etched through the apertures in the polycarbonate film to form corresponding openings in the non-insulating layer. As used here, xe2x80x9celectrically non-insulatingxe2x80x9d generally means electrically conductive or/and electrically resistive. The openings in the non-insulating layer can then be used to define locations for electron-emissive elements of an electron emitter. For example, the non-insulating layer can be a gate layer that overlies an electrically insulating layer. The insulating layer is etched through the openings in the gate layer to form dielectric open spaces in the insulating layer. Electron-emissive elements are formed in the dielectric open spaces.
When the polycarbonate film serves as a track layer in fabricating a gated electron emitter according to the foregoing process, providing the polycarbonate film with uniform thickness and uniform physical properties enables etching of the charged-particle tracks to be isotropic. As a consequence, the size of the gate openings created by using the aperture-containing polycarbonate track film varies little from opening to opening. The emission of electrons across the electron-emitting area of the electron emitter is quite uniform. A high quality electron-emitting device is thereby formed. In short, the invention provides a substantial technological advance over the prior art.